Technology: Surveillance device slips through the net

A system for covertly reading the number plates of cars at night has
been patented by the Ministry of Defence. The MOD does not normally publicise
this kind of sensitive research and vets all patent applications filed at
the British Patent Office. This one seems to have slipped through the security
net.

The patent application (number 2 248 994) can now be read by anyone,
in patent libraries anywhere in the world. When asked to comment the MOD
said: ‘If it’s to do with surveillance, then obviously we don’t want to
talk about it.’

Until now there has been no reliable way of watching either parked or
moving cars at night without the driver knowing. People under surveillance
seldom park conveniently under street lamps, and any attempt to beam light
on the number plate will give the game away.

It is possible to read the number plate with infrared illumination,
and a camera sensitive to infrared, but for anyone in the know there is
a simple trick to beat the system: you just switch on your vehicle headlights
to dazzle the sensor. The only way, so far, to produce an infrared beam
intense enough to compete with headlights has been to put an infrared filter
over an incandescent lamp. Anyone who knows what to look for, will spot
the source.

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The new system, which was developed by the MOD’s Defence Research Agency
in conjunction with Pearpoint, a Hampshire company, has a light source made
up from several hundred light-emitting diodes, of the type used singly in
the remote control units for televisions or video recorders. These LEDs
generate long-wavelength infrared, of up to 1000 nanometres, which is invisible
to the human eye.

Even 400 LEDs cannot normally defeat the dazzle from car headlights,
so the ganged array is fed a rapid stream of very short, intense electrical
pulses instead of a steady current. The LEDs can then handle ten times their
normal safe capacity of 100 milliamps, and briefly radiate ten times as
much light as they do if used continuously. The MOD favours using pulses
of 1 millisecond or less.

The infrared pulses are synchronised with a video camera. Conventional
video cameras shoot 50 pictures a second, each lasting 20 milliseconds.
The MOD’s camera also shoots 50 pictures a second but it is modified to
so that the exposure of each picture lasts 1 millisecond or less.

The camera is timed so that each picture coincides with an LED pulse.
Because the camera is only receiving light for 1 millisecond instead of
20 milliseconds, it cuts out 95 per cent of the light it would normally
receive from the headlights. But in that single millisecond it is receiving
the full dose of infrared light beamed from the LEDs and reflected by the
number plate. So headlights no longer swamp the infrared detector. Concentrating
the infrared pulse and reducing the exposure time even further makes surveillance
even easier. To maximise sensitivity, the camera lens is covered with a
filter which lets through mainly infrared light, and rejects light of other
wavelengths.

Because number plates in the UK are made to reflect light back in the
direction of its origin, the infrared light can be focused into a very
tight beam and placed close to the camera. This, and the long wavelength
of the light, make it very unlikely that anyone under surveillance will
notice a telltale light source.

The designers believe that the infrared source and camera can be packaged
in a cube just 10 centimetres square, and powered by a 12-volt battery.